By default, all Advanced Custom Fields metaboxes with a position/context of “side” are given a priority of “core” which means that they’ll be positioned below the WordPress Publish, Categories and Tags metaboxes, and possibly others. The function below can be used to bump up a priority of a metabox to “high” so that it appears at the very top of the side column (in two-column view).

Just be sure to change ‘My Field Group’ to the name of the Advanced Custom Fields field group whose priority you want to change.